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This animation shows the shapes of some elliptical orbits. These orbits have different eccentricities. If an ellipse has a high eccentricity, is it round like a circle or long like an oval?
Click on image for full size Original animation by Windows to the Universe staff (Randy Russell).

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Eccentricity of an Orbit

Do you think Earth moves around the Sun in
a circle? That is almost true, but not quite. The shape of Earth's orbit isn't
quite a perfect circle. It is more like a "stretched out" circle
or an oval. Mathematicians and astronomers call this shape an "ellipse".

Astronomers use a special word to describe how round, or how long and skinny,
and ellipse is. The word they use is "eccentricity". If the eccentricity of
an orbit is close to zero, the ellipse is nearly a circle. If it is close
to one, the ellipse
is
long
and skinny.

Earth's orbit has a small eccentricity, close
to zero. Earth's
orbit is almost
a perfect circle. Can you guess which planet has the highest eccentricity?
It's Pluto! Pluto's
orbit is not as much like a nice, round circle as Earth's.
Most comets have even more stretched out
orbits than planets. Comet orbits usually have high eccentricities.

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